Pears, unlike apples, do not ripen with good quality while still on the tree. They should be harvested when mature, not ripe. Mature pears are still too hard to eat, when they are ripe they are soft enough to eat. The maturity can be determined by looking closely at the skim of the pears as they approach harvest time (harvest time is the date when you harvested them with success in previous years). For most common varieties of pears, correct maturity is when the skin color changes from grass green to a lighter green and the lenticels (dots on the skin of the fruit) appear more prominent and start to spread apart. The skin of some varieties such as Bartlett (the most common canning pear) are lumpy when immature and become almost smooth when mature.
Another way to tell when pears are mature is the harvest test. When mature, pears usually detach from the tree easily when tilted to a horizontal position form their usual hanging position. This does not work for some varieties, Bosc is one variety (it has a long thin neck) that does not detach easily from the tree even when mature.
The pears in the top of the tree matures first, so they should be used as an indicators for the rest of the tree. In warm weather when the first few fruit are mature, the entire tree will be mature within a day or so. If undamaged pears start to drop, the entire tree should be harvested as soon as possible.
Please note: pears that have Codling Moth damage (the worm in the fruit) or damage from other insects or birds will ripen and fall from the tree before the other fruit on the tree are mature. Make sure that the fruit you are basing your decision to harvest on are not damaged.